Method for producing foamed polymer moulded bodies and foamed polymer moulded bodies

ABSTRACT

Method for producing foamed microporous polymer molded bodies by melting a thermoplastic polymer in a first zone of an extrusion device, mixing in a highly volatile blowing agent, conveying the polymer melt containing the blowing agent into a second zone in which dissolution of the blowing agent occurs to saturation of the polymer melt at the foaming temperature, and molding and foaming of the loaded polymer melt to a foamed structure, whereby, in the second zone, a pressure above 90 bar, a blowing agent concentration above the critical minimum concentration for complete foaming, and the foaming temperature, lying above the solidification temperature of the polymer melt saturated with blowing agent, are set such that the polymer molded body obtained has a porosity in the range between 40 and 90 vol. % and an open-cell pore structure with uniform cross-sectional distribution. Foamed microporous polymer molded bodies in the form of particles comprising a thermoplastic polymer with uniform open-cell pore structure, a porosity of 40 to 90 vol. %, an accessible proportion of pore volume of at least 0.75 and an average cell size between 1 and 100 μm.

This is a U.S. national stage application of International ApplicationNo. PCT/EP03/04500 filed Apr. 30, 2003. The entire disclosure of theprior application is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a method for producing a foamed open-cellmicroporous polymer molded body comprising at least one thermoplasticpolymer, and microporous foamed polymer molded bodies in the form ofpolymer particles of open-cell pore structure comprising at least onethermoplastic polymer.

2. Description of Related Art

Microporous polymer molded bodies find a variety of applications, inwhich they are used in a number of different forms. A wide area ofapplication of microporous polymer molded bodies in particle form is themasterbatch technique, in which open-cell porous polymer particles areloaded with liquid additives or even solid additives that are soluble inliquids. Examples of such additives are flame retardants, antioxidants,antistatics and plasticizers. The loaded polymer particles are thenmixed in the application as, for example, an additive masterbatch to abase polymer, by which means a homogeneous distribution of the additivesin the base polymer can be achieved even at very low concentration. Afurther application involves porous polymer particles loaded with activesubstances, whereby active substances such as fragrances orpharmaceutical active ingredients such as drugs are introduced into theporous structure of the particles and released into the environmentslowly and in a controlled manner in the application. The loadingcapacity of the porous polymer particles plays an important role inthese applications. For example, loading with additive or activeingredient of up to 70 wt. % relative to the total weight of polymer andadditive or active ingredient is sometimes required. Among theprerequisites for this is a high porosity of the polymer particles. Thepolymer particles must also possess sufficient stability, i.e., acompressibility that is as low as possible, otherwise, on storage of theparticles loaded with additives or active substances in, for example,containers or sacks, the additives or active substances may be releasedfrom the particles.

Finally, porous polymer particles can also be used, on account of theirlarge internal surface area, to absorb liquids, as for example in theseparation of oils from water.

Microporous polymer molded bodies in the form of hollow-fiber membranesor flat membranes find varied application in the filtration of fluids,particularly in the area of ultrafiltration and microfiltration. In thiscase it is attempted to obtain high porosities, in the absence of whichthe attainable throughput through the membranes is too low. On the otherhand the maximum porosity that can be set is often limited by therequirement that membranes used in processing and application have acertain minimum strength.

Various methods are known for production of microporous polymer moldedbodies. DE 27 37 745 C2, for example, describes a method for producingmicroporous polymer molded bodies, based on a process involvingthermally induced phase separation. In this method a homogeneoussolution of a polymer component in a suitable solvent system is firstproduced at elevated temperatures. The polymer components and thesolvent system form a binary system that in the liquid state ofaggregation has a region where it is present as a homogeneous solution,and another region in which it possesses a miscibility gap. Cooling ofsuch a system below the demixing temperature results in phase separationand finally in the formation of a porous polymer structure. Methods ofthis type for membrane production are described also in, for example,DE-A-32 05 289 and EP-A-0 133 882.

Another method for producing porous polymer particles is described in WO98/55540, in which a polyolefin polymer is dissolved in a solvent andthe solution dispersed, at a temperature above the crystallizationtemperature of the polyolefin, in a non-solvent for the polyolefin, withthe formation of a multiphase system. Porous polyolefin particles areobtained on cooling of the dispersion.

These known methods allow production of polymer molded bodies of highporosity and open-pored structure as well as high loading capacity.However, the above methods for their production have the disadvantagethat the required use of solvents necessitates costly extraction and/ordrying processes for removal of these solvents. Despite this costlyextraction and/or drying, complete removal of the solvent from thepolymer molded bodies obtained is usually not achieved, resulting inrestrictions on the use of such polymer molded bodies in the areas of,e.g., medicine or food technology, or even in the electrical industry.

Another method proposed for the production of porous polymer moldedbodies involves releasing the pressure on a pressurized melt consistingof a thermoplastic polymer containing a volatile blowing agent. U.S.Pat. No. 5,160,674, for example, describes a method for producing foamedmaterials from semicrystalline polymers, in which a pressurized melt ofthe polymer used is saturated with a gas and shaped, also underpressure; the reduction of pressure after removal from the die thencauses foaming of the polymer material. Foamed materials produced by themethod of U.S. Pat. No. 5,160,674 show a homogeneous porous structure,wherein the pores or cells are however closed. Closed-cell materials ofthis type cannot be loaded with additives or active substances, however,and are also unsuitable as membranes because they allow no throughput,or at best very low throughput, of the fluid to be filtered.

DE-A 44 37 860 describes a method for production of sheet-likemicrocellular foams from amorphous thermoplastic polymers such aspolystyrene, whereby a thermoplastic polymer melt is impregnated with avolatile blowing agent in a first extrusion zone and the melt containingthe blowing agent is then cooled in a second extrusion zone by at least40° C. to a temperature lying at least 30° C. above the glass transitiontemperature of the polymer containing the blowing agent. On releasingthe pressure on the melt to normal pressure and cooling it to roomtemperature, the melt expands and solidifies to a foam sheet.

WO 00/26006 describes a method for producing microcellular foams frompolymers or polymer mixtures for forming of molded bodies such ashollow-fiber or flat membranes. In a first extrusion zone a polymer meltis loaded with a compressed gas under the action of a shearing and/orkneading means, and in a second extrusion zone the solubility of the gasin the gas-loaded melt, and therefore foaming, are improved byincreasing the pressure. The foamed molded bodies obtained as aconsequence of pressure release after extrusion through a die may haveeither an open-cell or closed-cell structure depending on the setting ofthe process conditions, the mean cell size, according to the disclosedexample, lying in the range of approx. 10 μm. The foams obtained by themethod described in WO 00/26006 have high porosity in a range higherthan 90 vol. %. In many applications, however, foamed structures of thistype cannot be used on account of their poor mechanical stability.

WO 99/38604 discloses foamed porous membranes made from thermoplasticpolymers and a method for their production. The membranes described inWO 99/38604 have a mean pore diameter of between 0.05 and 30 μm, aporosity of at least 75 vol. % and a proportion of open cells of atleast 80%. They are produced by a method in which a polymer meltcomprising at least one polymer is conveyed through an extrusion deviceunder pressure and loaded with a cell former in an injection stage. Thepressure in this part of the extrusion device is set to at least 150 barand the temperature, which is above the glass transition or meltingtemperature, is so chosen that correct and smooth functioning of theextrusion device is guaranteed. In a downstream mixing step, asingle-phase melt is produced from the at least one polymer and the cellformer, the temperature of the melt in the mixing step being reducedbelow the working temperature in the previous part of the extrusiondevice and/or the pressure raised. The single-phase melt is extrudedthrough a die for shaping of the membrane, during which the cell formerfoams the polymer melt as a consequence of the resulting fall inpressure. To attain the desired proportion of open cells, the cellformers consist according to WO 99/38604 of at least two components,which are gases and/or low-boiling liquids that must have differentspeeds of diffusion relative to the polymer melt. The high proportion ofopen cells is ascribed, according to WO 99/38604, to the opening ofcells that were initially closed, apparently because the blowing agentwith the lower diffusion speed gives rise to a high internal cellpressure resulting in rupture of the cell walls, which are very thin onaccount of the high porosity also required. It is clear from this thatthe method of WO 99/38604 can be used at best to only a limited extentfor production of foamed structures of low porosity. Limitations arealso placed on the applicability of the method by the requirement thatthe cell former must consist of at least two components of differentdiffusion speeds relative to the polymer melt.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodby means of which microporous polymer molded bodies in the form of, forexample, particles or membranes can be produced economically without theneed for costly extraction procedures, whereby the polymer molded bodiesshould have a uniform pore structure, low compressibility and a highproportion of open cells.

It is a further object of the present invention to provide microporouspolymer molded bodies in the form of particles that are suitable forloading with additives or active substances and should possess uniformpore structure, low compressibility and a high proportion of open cells.

These and other objects are achieved on the one hand by a method forproducing foamed microporous polymer molded bodies comprising at leastone thermoplastic polymer, comprising the steps:

a. melting of the at least one thermoplastic polymer at a firsttemperature in a first zone of an extrusion device and introduction of ahighly volatile blowing agent, at a pressure higher than that of thepolymer melt so produced, into the first zone of the extrusion device,

b. loading of the polymer melt in the first zone of the extrusion devicewith the blowing agent, and mixing of the blowing agent into the polymermelt under the action of a shearing and/or kneading means on the polymermelt, whereby at least partial dissolution of the blowing agent in thepolymer melt occurs simultaneously,

c. conveying of the polymer melt loaded with blowing agent, by means ofa conveyor system coupled to a pressure regulating device, through asecond zone of the extrusion device connected to the first zone and intoa die positioned at the end of the second zone, whereby the temperaturein the second zone is set to a second temperature, defined as thefoaming temperature, which is equal to or lower than the firsttemperature, whereby dissolution of the blowing agent to saturation ofthe polymer melt occurs in the second zone,

d. molding of the polymer melt in the die and subsequent foaming of themolded polymer melt loaded with blowing agent on exit from the die ofthe polymer melt, to give a foamed structure on account of the blowingagent contained in the polymer melt,

e. cooling of the foamed structure until it solidifies,

whereby the polymer melt loaded with blowing agent produced in thesecond zone of the extrusion device has a solidification temperature,the method being characterized in that the pressure in the second zoneof the extrusion device is set above a minimum pressure ρ_(min) of 90bar, that the blowing agent concentration is set at least equal to thecritical minimum concentration for complete foaming, and that thefoaming temperature is set to a value above the solidificationtemperature such that the porous polymer molded body obtained has aporosity in the range between 40 and 90 vol. % and an open-cell porestructure with uniform cross-sectional distribution.

While state of the art methods lead to satisfactory results in the mainonly for very high porosities, the method of the invention allowsporosities to be set selectively over the entire range of 40 to 90 vol.% while retaining the combination of the features according to theinvention. The method of the invention thus produces polymer moldedbodies with a high proportion of open cells and at the same time greatmechanical stability. The foaming temperature for production of thepolymer molded bodies required by the invention is preferably set to amaximum of 30% higher than the solidification temperature, andespecially preferably to 10 to 30% higher than the solidificationtemperature.

Objects of the invention are also achieved by a method for production offoamed microporous polymer molded bodies comprising at least onethermoplastic polymer, comprising the steps:

a. melting of the at least one thermoplastic polymer at a firsttemperature in a first zone of an extrusion device and introduction of ahighly volatile blowing agent, at a pressure higher than that of thepolymer melt so obtained, into the first zone of the extrusion device,

b. loading of the polymer melt in the first zone of the extrusion devicewith the blowing agent, and mixing of the blowing agent into the polymermelt under the action of a shearing and/or kneading means on the polymermelt, whereby at least partial dissolution of the blowing agent in thepolymer melt occurs simultaneously,

c. conveying of the polymer melt loaded with blowing agent, by means ofa conveyor system coupled to a pressure regulating device, through asecond zone of the extrusion device connected to the first zone and intoa die positioned at the end of the second zone, whereby the temperaturein the second zone is set to a second temperature, defined as thefoaming temperature, which is equal to or lower than the firsttemperature, whereby dissolution of the blowing agent to saturation ofthe polymer melt occurs in the second zone,

d. molding of the polymer melt in the die and subsequent foaming of themolded polymer melt, on exit from the die of the polymer melt loadedwith blowing agent, to give a foamed structure on account of the blowingagent contained in the polymer melt,

e. cooling of the foamed structure until it solidifies,

whereby the polymer melt loaded with blowing agent produced in thesecond zone of the extrusion device has a solidification temperature,the method being characterized in that the pressure in the second zoneof the extrusion device is set above a minimum pressure ρ_(min) of 90bar, that the blowing agent concentration is set at least equal to thecritical minimum concentration for complete foaming, and that thefoaming temperature is set to a value that is 10 to 30% above thesolidification temperature.

It was found that this method allows production of porous polymer moldedbodies with a porosity in the range of 40 to 90 vol. % and an open-cellpore structure with uniform cross-sectional distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Cross-section of an apparatus suitable for carrying out themethod of the invention, in a simplified schematic representation.

FIG. 2: Dependence of the solidification temperature on CO₂concentration for the polypropylenes Stamylan 11E10 and Moplen VS6100K.

FIG. 3: Polymer molded body corresponding to Example 1 with inadequatefoaming on account of the blowing agent concentration being too low.

FIG. 4 Polymer molded body corresponding to Example 1 with completelyfoamed structure.

FIG. 5: Polymer molded body of the invention corresponding to Example 2,with a porosity of 74 vol. %.

FIG. 6: Polymer molded body of the invention corresponding to Example 4,with a porosity of 73 vol. %.

FIG. 7: Polymer molded body corresponding to Comparative Example 1 withcomplete foaming.

FIG. 8: Polymer molded body corresponding to Comparative Example 2 witha porosity of 72 vol. %.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The methods of the invention preferably can be used for production ofpolymer molded bodies with porosity in the range of 40 to 85 vol. %. Themethods of the invention are particularly suitable for production ofpolymer molded bodies with porosity in the range of 50 to 80 vol. %. Thefoaming temperature is therefore preferably set at a value 10 to 20%higher than the solidification temperature. The foaming temperature isespecially preferably set so that porous polymer molded bodies ofporosity between 60 and 75 vol. % are obtained. Furthermore, the methodsof the invention do not require extraction, which is usually costly, ofthe molded bodies obtained in order to remove residual solvents, forexample, so that the methods of the invention allow comparativelyeconomical production of microporous polymer molded bodies.

It has been found in connection with the invention that on plotting thesolidification temperature of the polymer melt, or of the polymer meltloaded with blowing agent, vs. the blowing agent concentration in thepolymer melt, two concentration regions can be distinguished. Thesolidification temperature of the polymer melt enriched or loaded withblowing agent is defined here as the lowest temperature at which correctfunctioning of the extrusion device in its second zone and in the die isguaranteed. Below the solidification temperature, the melt begins tosolidify and cannot exit from the die. Incipient die blockage can bedetected by an exponential increase of pressure in the second zone ofthe extrusion device. Once blockage has occurred, it cannot be removedby increasing the pressure but only by a considerable increase intemperature in the second part of the extrusion device. Thesolidification temperature depends on the type of polymer used and itsmolar mass or melt index as well as on the concentration of blowingagent dissolved in the melt.

As the blowing agent concentration increases, the solidificationtemperature in a first concentration region at first decreasesmonotonically and essentially linearly; in a second concentrationregion, at higher concentrations of blowing agent, it then becomesconstant and independent of blowing agent concentration. Theintersection of the best fit straight lines for these two regionsrepresents a critical blowing agent concentration. It is observed thatfor blowing agent concentrations below this critical concentration, theresulting polymer molded bodies have inhomogeneous distribution of cellswith some non-porous regions and macropores or cavities, as well asunsatisfactory porosity. In a concentration region above the criticalblowing agent concentration, on the other hand, complete foaming of thepolymer molded bodies can be achieved. In the context of the presentinvention, complete foaming is understood to be foaming in which thepolymer molded body has a porosity of at least 95 vol. % and alsohomogeneous distribution of the pores or cells. A homogeneous poredistribution is one in which the pore structure is uniform with nomacropores or cavities, and in which the diameter of the largest poresdiffers from the mean pore diameter by a factor of 10 at most. Thecritical blowing agent concentration therefore corresponds to thecritical minimum concentration for complete foaming.

It was now found that in the method of embodiments of the invention, theblowing agent concentration, i.e., the concentration, relative to themelt comprising the at least one polymer, of the blowing agentintroduced into the first zone of the extrusion device, must be set to avalue at least equal to the critical minimum concentration for completefoaming. This also ensures that the proportion of open cells required bythe invention in the polymer molded body is attained. On the other hand,it is not practical to set the blowing agent concentration at too high avalue. At blowing agent concentrations that are significantly too high,it is no longer possible to dissolve the entire quantity of blowingagent in the melt. In such cases, fairly large quantities of undissolvedblowing agent can escape from the die; in addition, large zones ofundissolved blowing agent in the melt could lead to formation ofmacropores. The blowing agent concentration should therefore preferablyexceed the critical blowing agent concentration, i.e., the minimumconcentration for complete foaming, by a maximum of 5 wt. % andespecially preferably by a maximum of 3 wt.%.

In the method of embodiments of the invention, a melt of the at leastone thermoplastic polymer introduced into the extrusion device is firstproduced in the first zone of the extrusion device, and this melt isconveyed through the extrusion device under pressure. The introductionof the highly volatile blowing agent into the polymer melt occurspreferably under a pressure higher than that in the first zone of theextrusion device, and advantageously at a temperature higher than theglass transition or melt temperature of the at least one polymer in thefirst zone of the extrusion device. The blowing agent can be introducedfor example in an injection step, e.g., through a sintered metal, toachieve good distribution of the blowing agent in the polymer melt. Theblowing agent is preferably introduced at a pressure greater than 150bar. Loading of the polymer melt with the blowing agent introduced andmixing of the blowing agent into the polymer melt occur under the actionof a shearing or kneading means on the polymer melt. The temperature inthis area of the extrusion device is preferably so chosen that,immediately after introduction of the blowing agent, further treatmentof the polymer melt loaded with blowing agent occurs while maintainingthe temperature of the melt of the at least one polymer in its pureform, or at temperatures lower than this. By this means alone, part ofthe blowing agent dissolves in the polymer melt.

The highly volatile blowing agent used in the context of the method ofthe invention should be at least largely inert towards the at least onepolymer used. The highly volatile blowing agent can be a low-boilingliquid or a gas. It is also possible to use mixtures of variouslow-boiling liquids, mixtures of various gases, and even mixtures of,for example, a low-boiling liquid and a gas, as described for example inWO 99/38604. Examples of the low-boiling liquids that can be used arewater, pentane, and even low-boiling alcohols. Suitable gases includeethane, propane, butane, nitrogen and carbon dioxide. Carbon dioxide isused in preference.

The method of embodiments of the invention can be used for amorphous andsemicrystalline thermoplastic polymers. Suitable polymers includeparticularly polyolefins, polyesters, sulfone polymers such aspolysulfone or polyethersulfones, polyamides and polycarbonates, as wellas modifications, blends, mixtures and copolymers of these polymers.Polyolefins such as polyethylene, polypropylene, polybutylene andpoly(4-methyl-1 -pentene) are preferably used, and polypropyleneespecially preferably used.

The polymer melt loaded with blowing agent is conveyed into the secondzone of the extrusion device by a conveyor system in the form of, forexample, a gear pump or a second extruder, coupled to a pressureregulating device. This second zone of the extrusion device ispreferably designed as an additional mixing stage. In a preferredembodiment, the second zone is a tubular extruder head that can beheated and also cooled if necessary, which in an especially preferredembodiment is provided with mixing elements, for example in the form ofstatic mixers. In the second zone of the extrusion device, saturation ofthe polymer melt with the blowing agent should occur and the blowingagent should be dissolved as completely as possible in the polymer melt.It is therefore advantageous for the polymer melt that has been loadedwith blowing agent to be mixed in the second zone by mixing elements,which are preferably in the form of static mixers. The foamed polymermolded bodies obtained in this way show a high degree of homogeneity. Atthe end of the second zone, a further pressure regulating device such asanother melt gear pump can be interposed before the die, to allow thepressure in the second zone of the extrusion device to be set withflexibility.

The division of the extrusion line into a first zone and a second zonethat is decoupled from the first allows the temperature and pressure ineach zone to be set as required by the current process and independentlyof the temperature and pressure in the other zone. It is thereforepossible to reduce the temperature in the second zone, i.e., the foamingtemperature, relative to that in the first zone, and/or to increase thepressure in the second zone relative to that in the first zone. This isadvantageous because lower temperatures and higher pressures allow ahigher blowing agent concentration to be introduced into the meltwithout undissolved blowing agent remaining in the melt. Reduction ofthe temperature of the melt loaded with blowing agent and/or increase ofpressure in the second zone of the extrusion device allows more blowingagent to be introduced into the polymer melt in the first zone than canbe dissolved under the conditions prevailing in the first zone, becausethe excess is brought into solution subsequently, during treatment inthe second zone of the extrusion device. The effect can be exploitedhere of the lowering of the softening or melting temperature of thepolymer melt loaded with blowing agent, or of the reduction in viscosityof the melt at a given temperature, as the blowing agent contentincreases. It is therefore possible to reduce the processing or foamingtemperature in the second zone of the extrusion device relative to thetemperature in the first zone without any increase in viscosity. Inaccordance with one aspect of the invention, however, the foamingtemperature must be set to a value that is 10 to 30%, and preferably 10to 20%, higher than the solidification temperature of the polymer meltloaded with blowing agent in the second zone.

The pressure in the second zone of the extrusion device is preferablyhigher than that in the first zone. According to embodiments of theinvention, the pressure in the second zone must be at least 90 bar, andis preferably set to values greater than 120 bar and especiallypreferably to values greater than 200 bar. This also allows stableprocess control.

After passing through the second zone of the extrusion device, thepolymer melt loaded with blowing agent is molded in a die connected tothe second zone, and on leaving the die is foamed to give the foamedstructure as a result of the reduction in pressure, typically to normalpressure. It is advantageous if the temperature of the die can be setindependently of that in the second zone of the extrusion device. Thedie is preferably conditioned to a die temperature that is independentof the foaming temperature and lies above it. The die temperature isespecially preferably up to 20% higher than the foaming temperature. Theporosity can thus be influenced in the direction of a reduction inporosity as compared with a polymer molded body in the production ofwhich the foaming temperature and die temperature are the same.

Foamed structures and therefore polymer molded bodies can be produced invarious forms, depending on the die used. In preferred embodiments,foamed polymer molded bodies in the form of a solid fiber, hollow fiberor flat sheet are produced by the method of the invention. Foamedpolymer molded bodies in the form of a hollow fiber or flat sheet areespecially preferably hollow-fiber membranes or flat membranes. When ahollow fiber or hollow-fiber membrane is being produced, the polymermelt loaded with blowing agent is extruded through a die in the form ofa hollow-fiber die with a central capillary, whereby for example a gasacting as a lumen filler is metered through the interior bore of thecapillary to form and stabilize the cavity of the hollow fiber beingproduced.

In another preferred embodiment of the method of the invention, thefoamed structure is broken up in an additional process step to givepolymer molded bodies in the form of foamed polymer particles. Thecooled foamed structure is preferably granulated by, for example, acutting disk, granulator, blade or fly cutter to give polymer moldedbodies in the form of foamed polymer particles. For the breaking up, thepreviously stabilized foamed structure is preferably further cooled by,for example, ice water, liquid nitrogen or dry ice to guarantee a highproportion of open cells at the interfaces.

The method of the invention is therefore excellently suited forproduction of microporous polymer particles for use as, for example, anadditive masterbatch or for loading with active substances, the polymerparticles having low compressibility and high loading capacity. Becauseno solvent is used in the method of the invention, this method allowsthe production of microporous polymer particles that are especiallysuitable also for use for in medical applications.

The invention therefore relates also to foamed microporous polymermolded bodies in particle form, i.e., foamed microporous polymerparticles made from at least one thermoplastic polymer with open-cellpore structure distributed uniformly over the particle cross-section, aporosity in the range of 40 to 90 vol. %, and an accessible proportionof pore volume of at least 0.75, the cells forming the open-cell porestructure having a mean size of between 1 and 100 μm.

The foamed microporous polymer particles are especially suitable forloading with additives or active substances.

The accessible proportion of the pore volume, or pore accessibility,which is important with respect to loadability, is understood here to bethe proportion of the pore volume provided by the porosity that isaccessible to an additive or an active substance and can therefore beloaded with an additive or an active substance. The accessibleproportion of the pore volume is therefore also a measure of theproportion of open cells of the structure of the polymer molded body ofthe invention or produced by the method of the invention. The accessibleproportion of the pore volume is preferably at least 0.85.

For the use of the polymer particles of the invention, it is importantfrom the economic viewpoint that, apart from the general accessibilityof the pores, additives or active substances can be taken up by thepolymer particles within a sufficiently short time. Polymer particles ofthe invention preferably have a characteristic loading time of <90 minand especially preferably of <45 min. The particles can therefore beloaded with additives or active substances in a sufficiently short time.In the context of the present invention, the characteristic loading timeis understood as the time required by the polymer molded body to absorbthe quantity of additive necessary for loading of 90% of the accessiblepore volume. The accessible proportion of the pore volume as currentlydetermined for the polymer particle in question is used for thispurpose. A silane of type Silcat XL70, commercially available from theOrgano Silicones Group of Witco Surfactants GmbH, Germany, is used inthe present invention as additive for determination of poreaccessibility and characteristic loading time.

Microporous polymer particles having a porosity in the range 40 to 85vol. % are preferred, and those with a porosity between 50 and 80 vol. %are especially preferred. Microporous polymer particles with a porositybetween 60 and 75 vol. % have especially well-balanced properties. Alsopreferred are microporous polymer particles having a mean cell sizebetween 5 and 50 μm.

The structure of the polymer particles of the invention, or of thepolymer molded bodies produced by the method of the invention, isdistinguished by a three-dimensional network of cells or honeycombsseparated from one another by thin walls, the cells or honeycombs beingconnected with one another via holes or perforations or by permeablenetwork structures in the walls. Permeable network structures of thistype that connect cells or honeycombs are characteristic of the presentpolymer molded bodies or polymer particles, and are presumably due tobursting of thin wall structures during foaming of the polymer melt thatstill possesses plastic deformability.

The general criteria for assessing whether polymer particles aresuitable for loading with active substances or for use as an additivemasterbatch are primarily the porosity and the proportion of open cellsof the polymer particles. Adequately high porosity of the polymerparticles is a prerequisite for the uses mentioned above. At the sametime, the proportion of open cells of the pore structure must be high,the crucial factors here being not only the existence of connectionsbetween the cells constituting the foamed structure but also adequatepermeability of these connections to the additive or active substancewith which the polymer particles are to be loaded. A high loadingcapacity of the particles is also an important assessment criterion. Itis furthermore important, for economy of processing, that the foamedpolymer particles be capable of being loaded with additive or activesubstance within a relatively short time. Finally, adequate stability,i.e., a compressibility that is the minimum possible, is an importantcriterion from the viewpoint of storage of the loaded polymer particles.The requirements are excellently met by the polymer particles of theinvention or produced by the method of the invention.

In a further preferred embodiment, the polymer particles of theinvention have a mean particle size in the range of 1 to 5 mm.

Good results are obtained for the loading capacity of the polymerparticles of the invention when the particle cells on the externalsurface are accessible. It is therefore advantageous if the polymerparticles of the invention have a surface porosity, averaged over allexternal surfaces, of at least 25%.

On account of their well balanced structure, e.g., their definedporosity in the range between 40 and 90 vol. % and a mean cell size ofless than 100 μm and preferably less than 50 μm, the foamed polymerparticles of the invention have high stability, i.e. lowcompressibility. In use, therefore, polymer particles of the inventionloaded with additive or active substance can safely be stored withoutdanger of release of the additive or active substance from the porestructure as a result of compression of the particles.

The polymers from which the polymer particles of the invention are madecan be the same as those used in the method of the invention forproduction of the polymer particles of the invention. The polymerparticles of the invention preferably comprise at least one polyolefinand especially preferably a polypropylene.

The invention will now be explained in detail with the help of thefigures and examples of embodiments.

FIG. 1 is a schematic representation of an apparatus suitable forcarrying out the method of embodiments of the invention. The apparatus10 shown comprises essentially an extruder 11, forming the first zone150 of the extrusion device, with a long extruder body.Shearing/kneading/homogenizing devices 16 of the type, for example, of ahelical conveyor that is known per se, are mounted in a way that isknown per se in this extruder 11. The extruder has a funnel-shaped inlet110 through which the at least one polymer 13, typically in the form ofa granulate or powder, is fed into the extruder 11 and then conveyed asa melt to an outlet 111 located opposite the inlet 110 by means of theshearing/kneading/homogenizing devices 16. The apparatus has for thispurpose a drive motor 19 and if required a gear mechanism 20, by whichthe shearing/kneading/homogenizing devices 16 are rotatably coupled tothe drive motor 19.

Mounted all around the long cylinder of the extruder 11 aretemperature-control devices 18, which can be cooling devices or heatingdevices. In the central section of the extruder 11, the blowing agentused for foaming is injected through a dosing system and supply line 14into the interior of the extruder. A conveyor system 17, for example inthe form of a melt gear pump, coupled to a pressure regulating device issituated directly next to the outlet 111 and therefore forms the end ofthe first zone of the extrusion device.

Adjacent to the conveyor system 17 is the second zone 151 of theextrusion device, which in the present embodiment is in the form of, forexample, a tubular extruder head 22 with a right-angle bend. The secondzone is also equipped with temperature-control devices 18, by means ofwhich the temperature of the gas-enriched polymer melt located in theinterior of the extruder head can be appropriately regulated. Theextruder head 22 preferably contains mixing devices, which are not shownin the diagram, static mixing elements being excellently suited for thepurpose. At the end of the extruder head 22 as viewed in the directionof extrusion is a die 12, by means of which the polymer melt loaded withblowing agent is molded. On exit from this die, the polymer melt loadedwith blowing agent is foamed to give a porous molded body. The exit ofthe molded body from the apparatus 10 is symbolized by the arrow 21.

The method of embodiments of the invention is carried out as followswith the use of the apparatus 10 described above.

The at least one polymer 13, preferably in the form of a granulate, isfed through the funnel-shaped inlet 110 into the extruder 11 that formsthe first zone 150 of the extrusion device, whereby the at least onepolymer is first melted by means of the temperature-control devices 18.The melting and finally melted polymer is conveyed by means of theshearing/kneading/homogenizing devices 16, which are driven by drivemotor 19, to the region of the extruder 11 into which the blowing agentis injected under high pressure via the dosing system and supply line 14into the interior of the extruder 11. The quantity of blowing agent isso adjusted that the concentration of blowing agent in the polymer meltlies above the minimum concentration for complete foaming.

The rotation of the shearing/kneading/homogenizing devices 16 preventsthe blowing agent from being deposited on the surface of the melt of theat least one polymer. It also effects mixing of the blowing agent intothe polymer melt, whereby the blowing agent dissolves at least partiallyin the polymer melt. In the part of the extruder 11 downstream of theinlet feed for the blowing agent, the temperature of the melt enrichedwith blowing agent can be lowered relative to the original melttemperature and then held essentially constant up to outlet 111 by meansof the temperature-control devices 18, to increase the solubility of theblowing agent in the polymer melt.

The polymer melt loaded with blowing agent is conveyed to the secondzone 151 of the extrusion device by means of the melt gear pump 17. Inthis second zone, which in the present invention is in the form of atubular extruder head 22, the temperature of the polymer melt loadedwith blowing agent is reduced to the foaming temperature by means of thetemperature-control devices 18 mounted there, the foaming temperaturedepending on the desired porosity of the porous polymer molded bodyobtained, and being preferably up to 30% higher than the solidificationtemperature of the polymer melt loaded with blowing agent. The pressurein the second zone 151 may simultaneously be increased if required, theminimum pressure being 90 bar in every case. These conditions furtherincrease the solubility of the blowing agent in the polymer melt.

The polymer melt loaded with blowing agent is molded in the die 12 and,on leaving die 12, foams to a foamed structure on account of thereduction in pressure. The foamed structure is then cooled in anappropriate manner to obtain the porous polymer molded body of theinvention with an open-cell structure and a porosity that can be set inthe range 40 to 90 vol. %.

In the examples below, the following methods were used forcharacterization of the porous polymer molded bodies obtained.

Determination of Particle Size

The average particle size can be determined microscopically with thehelp of a representative quantity of the sample, using a measuringeyepiece or an appropriate image analysis method.

Determination of Mean Cell Size

The mean cell size or pore size is determined with the help ofdigitalized SEM micrographs of fracture patterns of the samples whichare analyzed with the help of suitable image analysis software. A SEMmicrograph allows measurement in μm of the pore diameter or celldiameter of approx. 50 to 100 cells or pores. The mean cell size or meanpore diameter is then calculated from the individual values byaveraging.

Determination of Volume Porosity

The volume porosity is determined pycnometrically. Approx. 1 to 5 g ofthe test material is weighed-in dry, the test material having previouslybeen broken up if required by means of a blade, for example, and withcooling. To prevent floating of the test material during the subsequentmeasurement on account of the low density of the porous particles, thetest material is introduced into the pycnometer in a suitable cage, a100 ml pycnometer with thermometer and side capillary being used. Thepycnometer is then filled with a suitable measurement liquid that doesnot wet the test material; water (18 MOhm-water) can generally be usedfor this purpose. The actual volume of the pycnometer is determined inadvance, by use of the same measurement liquid that is subsequentlyused. The measurement is carried out at 20° C. The porosity ε of thetest material can be determined from the initial weight of testmaterial, the density of the polymer constituting the test material, thedensity of the measurement liquid used and the difference in mass of themeasurement liquid in the pycnometer with and without the test material.

Determination of the Accessible Proportion of the Pore Volume, or PoreAccessibility

Determination of the accessible proportion of the pore volume or poreaccessibility requires that the volume porosity of the test material beknown.

Approx. 10 to 30 g of the test material are weighed into a 500 ml glassflask. If the test material consists of polymer molded bodies in theform of strands or films, these are first broken up into particles ofdimensions in the range of approx. 1 to 5 mm. Care must be taken thatopen cut faces are obtained during this breaking up, which isconveniently carried out by means of sharp tools such as razor blades,microtome blades or granulators and, if required, with cooling. If thetest material is already in particle form, it can be used directly.

The accessible proportion of the pore volume is determined by measuringthe loading capacity by means of an additive, the additive used being asilane of type Silcat XL70 (from Witco Surfactants GmbH, OrganoSilicones Group). The quantity, i.e., volume, of the silane that is tobe added to the test material is determined by the porosity of the testmaterial or by the pore volume of the test material initiallyweighed-in, the pore volume of the test material being determined fromthe initial weight, the polymer density ρ_(polymer) and the porosity ε.In the first step, silane is metered in a quantity that can be expectedto be completely absorbed by the test material. The volume of the silaneadded corresponds to approx. 60% of the pore volume determined for thesample.

After the addition, the glass flask is attached to a suitable mixingdevice such as a rotary evaporator with a water bath maintained at atemperature of 25° C. Mixing is continued until the test material is dryfrom the outside and flows freely. The loading time from the start ofmixing to complete absorption of the additive is determined with astopwatch.

The glass flask is then detached from the mixing device and a furtherquantity of silane, corresponding to 5% of the pore volume, is added.Mixing is then performed again, and the time required for this quantityof silane to be completely absorbed by the sample is recorded. Thisprocess is repeated until the test material is saturated with theadditive, the quantity of silane metered in each time corresponding to5% of the pore volume. Saturation is defined here as the state in whicheven after a total loading time of 3 hours, a film of the silane remainson the wall of the glass flask and/or agglutination of the particles ofthe test material is observed. The total loading time is defined here asthe sum of the individual loading times as determined with thestopwatch. The saturated test material is then reweighed and the totalquantity of silane absorbed by the test material is determined bysubtracting the initial weight from the current weight.

The accessible proportion of the pore volume or pore accessibility α canthen be found by substituting the density ρ_(silane) of the SilcatXL-Pearl 70 silane used (ρ_(silane)=0.91 g/cm³) in the equation:$\alpha = {\frac{V_{silane}}{V_{{pore}\quad{volume}}} = \frac{m_{silane}/\rho_{silane}}{\frac{ɛ}{100 - ɛ} \cdot \left( {m_{test}/\rho_{polymer}} \right)}}$where

-   α=pore accessibility or accessible proportion of pore volume,-   V_(silane)=volume in cm³ of the silane added until saturation of the    test material occurs,-   V_(pore volume)=pore volume in cm³ of the weighed-in test material,-   m_(silane)=mass in g of silane added until saturation of the test    material occurs,-   m_(test)=initial weight in g of the test material,-   ρ_(silane)=density in g/cm³ of the silane used,-   ρ_(polymer)=density in g/cm³ of the polymer constituting the test    material,-   ε=porosity (as a percentage) of the test material

Determination of the Characteristic Loading Time

The determination of the characteristic loading time assumes that theporosity ε and the accessible pore volume a of the test material areknown.

To determine the characteristic loading time, a quantity of testmaterial between approx. 10 and 30 g is weighed into a 500 ml glassflask. The sample is prepared as described above.

The characteristic loading time is determined for the quantity ofadditive necessary for loading of 90% of the accessible pore volume ofthe test material. The silane Silcat XL-Pearl 70 is again used as theadditive. The quantity of silane m_(90%) to be added to the weighed-intest material can be determined from the equation:${m_{90\%}\lbrack g\rbrack} = {0.9 \cdot \alpha \cdot \rho_{silane} \cdot \frac{ɛ}{100 - ɛ} \cdot \left( {m_{test}/\rho_{polymer}} \right)}$After addition of the silane, the glass flask is attached to a suitablemixing device such as a rotary evaporator with a water bath maintainedat a temperature of 25° C. Mixing is continued until the test materialis dry from the outside and flows freely. The period of time from thestart of mixing to complete absorption of the additive is measured witha stopwatch and represents the characteristic loading time of the testmaterial.

Determination of Surface Porosity

The surface porosity is determined by means of scanning electronmicrographs (SEM). SEM micrographs of approx. 100-fold magnification areobtained of representative sections of the particle surface of size 0.8mm×0.6 mm, and the images are analyzed. The SEM micrographs aredigitalized and the area of the dark regions on the micrograph, whichcorrespond to the pores or cells and stand out from the lighter regionsascribed to the cell walls, are determined with the help of a computer.The surface porosity is then calculated as the ratio of the pore area tothe total surface area.

Determination of Compressibility

The measurement is performed for solid fibers or hollow fibers by meansof a commercially available semi-automatic thickness tester that issuitable for testing in accordance with DIN 53855 Part 1, such as adevice of type 16304 from Karl Frank GmbH. In this device a piston witha variable loading weight can be lowered on to, and raised from, thesample under measurement by a geared motor. The displacement is measuredby means of a digital displacement sensor.

A piston of area 10 cm² and diameter 35.68 mm is used for themeasurement. A straight piece of the test material is fixed on the baseplate in such a way that the piston can press down radially on thecylindrical surface of the test material with its entire diameter, alonga line of length 35.68 mm. The test sample is subjected to various loadsat room temperature and the distance between the lower edge of thepiston and the base plate is measured in every case after a load time of10 sec. Contact forces or loads of, e.g., 0.5, 1.25, 2.5, 5.5, 12.5, 20,25 and 50N, are applied in an ascending sequence.

The quotient of the measured distances, i.e., of the respectivethicknesses of the test material in the compressed state to the originalsample thickness in the uncompressed state, is plotted on asemilogarithmic scale against the applied force. The measured distancefor a load of 0.5N is taken as the original sample thickness. From thequotient Q for a load of 10N, the percentage compressibility of the testmaterial is determined as (1−Q)·100.

EXAMPLE 1

To determine the critical minimum concentration for complete foaming,the polypropylenes Stamylan 11E10 (from DSM; melt index=0.3 g/10 min at230° C./21.6N) and Moplen VS 6100K (from Montell; melt index=25 g/10 minat 230° C./21.6N) were fed into an extrusion device of the type shown inFIG. 1 and melted in the extruder. A melt temperature of 250° C. was setfor Stamylan 11E10 and of 185° C. for Moplen VS 6100K. Supercritical CO₂was injected as blowing agent into the molten polypropylene melt in aconcentration between 0 and 13 wt. % and at a pressure in the range of160 to 210 bar. Following the injection, the CO₂ was mixed into thepolymer melt by the kneading and shearing action of the extruder screw,and partially dissolved in the melt.

The respective polypropylene melt loaded with CO₂ was conveyed by meansof a melt gear pump to the second zone of the extrusion device, in theform of an angled tubular extruder head, and then removed from theextrusion device via a nozzle with circular outlet attached to the endof the second zone. The gas-loaded polymer melt foamed in the process toa foam fiber. By means of temperature-control elements mounted on theextruder head, the temperature of the gas-loaded melt was in each casereduced stepwise to the solidification temperature, i.e., thetemperature below which solidification of the CO₂-loaded polypropylenemelt occurs and the melt no longer emerges from the nozzle.

The solidification temperatures for the two types of polypropyleneinvestigated are plotted in FIG. 2 as a function of blowing agentconcentration in wt. %, i.e., the quantity of CO₂ metered into theextruder per time unit, relative to the amount of polymer solutiontransported in the time unit (and therefore relative to the totalquantity of polymer and blowing agent).

For both the polymers, in a first concentration range, thesolidification temperature initially decreases relative to thesolidification temperature of the pure polymer as the CO₂ concentrationincreases, and then, in a second concentration range, remains constantas the CO₂ concentration is further increased. The regression linesdrawn through the points in each region intersect at approx. 7.7 wt. %for Stamylan 11E10 and at approx. 10.5 wt. % for Moplen VS 6100K. TheCO₂ concentrations associated with these points of intersectionrepresent the critical minimum concentrations for complete foaming forthe Stamylan 11E10/CO₂ and Moplen VS 6100K/ CO₂ systems. At CO₂concentrations below the critical minimum concentration for completefoaming, fully foamed structures are not obtained (FIG. 3). It is onlyabove the critical minimum concentration that complete foaming isattained, with a porosity above 95 vol. % and homogeneous pore structure(FIG. 4).

EXAMPLE 2

Polypropylene of the type Stamylan 11E10 (from DSM) was processed in anextrusion device as in Example 1. The polymer was melted at atemperature of 250° C. Supercritical CO₂ was injected as blowing agentinto the melted polymer at a concentration of 7.7 wt. % and a pressureof 179 bar. After passing through the second zone, the CO₂-loadedpolypropylene melt was extruded through a nozzle of diameter 0.8 mm. Thetemperature in the second zone, i.e., the foaming temperature, was setto 188° C., which is approx. 15% higher than the solidificationtemperature of 163° C. The pressure in the second zone was higher than97 bar.

On leaving the nozzle, the extruded melt foamed to a porous solid fiberwith microporous, open-cell structure and a diameter of 2.25 mm. Theporosity was determined as 74 vol. %. The foamed solid fiber had a meancell size of approx. 34 μm (FIG. 5 a,b). The compressibility of thissolid fiber under a load of 10N was 15%.

In the use of this type of product as, for example, an additiveconcentrate, a maximum compressibility of 25% under a load of 10N isregarded as adequate to guarantee sufficient storage stability.

EXAMPLE 3

The porous Stamylan 11E10 solid fiber of Example 2 was broken up, withcooling, by means of a granulator to give cylinder-shaped particles oflength approx. 2.25 mm and a diameter corresponding to that of the solidfiber. For a total porosity of 74 vol. %, the porous polymer particlesso produced had a pore accessibility a of 0.87 and a characteristicloading time of 35 min. Their surface porosity was approx. 29%.

EXAMPLE 4

Polypropylene of type Moplen VS 6100K (from Montell) was processed in anextrusion device as in Example 1. The polymer was melted at atemperature of 185° C. Supercritical CO₂ was injected as blowing agentinto the melted polymer at a concentration of 10.5 wt. % and a pressureof 210 bar. After passing through the second zone, the CO₂-loadedpolypropylene melt was extruded through a nozzle of diameter 0.8 mm. Thetemperature in the second zone was set to 164° C., which is approx. 11%higher than the solidification temperature of 148° C. The pressure inthe second zone was approx. 112 bar.

On leaving the nozzle, the extruded melt foamed to a porous solid fiberwith a microporous, open-cell structure and a diameter of 1.15 mm. Theporosity was determined as 73 vol. %. The foamed solid fiber had a meancell size of approx. 44 μm (FIG. 6 ). The compressibility of this solidfiber under a load of 10N was 18%.

EXAMPLE 5

The porous Moplen VS 6100K 6100 solid fiber of Example 4 was broken up,with cooling, by a granulator to give cylinder-shaped particles oflength approx. 1.5 mm and a diameter corresponding to that of the solidfiber. For a total porosity of 73 vol. %, the porous polymer particlesso produced had a pore accessibility α of 0.85 and a characteristicloading time of 20 min. Their surface porosity was approx. 27%.

COMPARATIVE EXAMPLE 1

Analogously to Example 2, polypropylene of type Stamylan 11E10 wasprocessed in an extrusion device as described for Example 1.Supercritical CO₂ in a concentration of approx. 8 wt. % was injectedinto the melted polymer. The temperature in the second zone was,however, set to 172° C., which is approx. 5.5% higher than thesolidification temperature of 163° C.

The foamed solid fiber so obtained had a porosity of 96 vol. %, whichcorresponded to the porosity for complete foaming and lay outside therange required by the invention (FIG. 7). The set foaming temperature of172° C. was therefore not sufficiently high. This fully foamed solidfiber, and therefore also the porous polypropylene particles producedfrom it, had a compressibility under a load of 10N of 39%, which isclearly too high.

COMPARATIVE EXAMPLE 2

Again analogously to Example 2, polypropylene of type Stamylan 11E10 wasprocessed in an extrusion device as described for Example 1.Supercritical CO₂ was injected as blowing agent into the melted polymer,but in a concentration of only 4.3 wt. %. The temperature in the secondzone was set to 179° C., which is slightly above the solidificationtemperature determined for this concentration.

The foamed solid fiber thus obtained, while having a porosity of 72 vol.%, showed in cross-section a marked inhomogeneity in pore structure withlarge non-porous areas (FIG. 8).

1. A method for production of foamed microporous polymer molded bodiescomprising at least one thermoplastic polymer, comprising the steps a.melting of the at least one thermoplastic polymer at a first temperaturein a first zone of an extrusion device, and introduction of a blowingagent, at a pressure higher than that of the polymer melt so produced,into the first zone of the extrusion device, b. loading of the polymermelt in the first zone of the extrusion device with the blowing agent,and mixing of the blowing agent into the polymer melt under the actionof a shearing and/or kneading device on the polymer melt, whereby atleast partial dissolution of the blowing agent in the polymer meltoccurs simultaneously, c. conveying of the polymer melt loaded withblowing agent, by a conveyor system coupled to a pressure regulatingdevice, through a second zone of the extrusion device connected to thefirst zone and into a die positioned at the end of the second zone,whereby the temperature in the second zone is set to a secondtemperature, defined as the foaming temperature, which is equal to orlower than the first temperature, whereby dissolution of the blowingagent to saturation of the polymer melt occurs in the second zone, d.molding of the polymer melt in the die and subsequent foaming of themolded polymer melt loaded with blowing agent, on exit of the polymermelt from the die, to give a foamed structure on account of the blowingagent contained in the polymer melt, e. cooling of the foamed structureuntil its solidification, whereby a pressure greater than the minimumpressure ρ_(min) of 90 bar is set in the second zone of the extrusiondevice and the polymer melt loaded with blowing agent produced in thesecond zone of the extrusion device has a solidification temperature,the method being characterized in that, a blowing agent concentration isset that is at least equal to a critical minimum concentration forcomplete foaming, and the foaming temperature is set to a value abovethe solidification temperature such that the porous polymer molded bodyobtained has a porosity in the range between 40 and 90 vol. % and anopen-cell pore structure with uniform cross-sectional distribution. 2.Method according to claim 1, wherein the foaming temperature is set to avalue that is up to 30% higher than the solidification temperature. 3.Method according to claim 1, wherein the foaming temperature is set to avalue that is 10 to 30% higher than the solidification temperature. 4.Method according to claim 1, wherein the foaming temperature is set to avalue 10 to 20% higher than the solidification temperature, such thatthe porous polymer molded body obtained has an open-cell structure and aporosity in the range of 50 to 80 vol. %.
 5. Method for production offoamed microporous polymer molded bodies comprising at least onethermoplastic polymer, comprising the steps a. melting of the at leastone thermoplastic polymer at a first temperature in a first zone of anextrusion device, and introduction of a blowing agent, at a pressurehigher than that of the polymer melt so produced, into the first zone ofthe extrusion device, b. loading of the polymer melt in the first zoneof the extrusion device with the blowing agent, and mixing of theblowing agent into the polymer melt under the action of a shearingand/or kneading device on the polymer melt, whereby the blowing agentsimultaneously dissolves at least partially in the polymer melt, c.conveying of the polymer melt loaded with blowing agent, by a conveyorsystem coupled to a pressure regulating device, through a second zone ofthe extrusion device, connected to the first zone, to a die positionedat the end of the second zone, whereby the temperature in the secondzone is set to a second temperature, defined as the foaming temperature,which is equal to or lower than the first temperature, wherebydissolution of the blowing agent occurs to saturation of the polymermelt in the second zone, d. molding of the polymer melt in the die andsubsequent foaming of the molded polymer melt loaded with blowing agent,on exit of the polymer melt from the die, to give a foamed structure onaccount of the blowing agent contained in the polymer melt, e. coolingof the foamed structure until its solidification, whereby a pressuregreater than the minimum pressure ρ_(min) of 90 bar is set in the secondzone of the extrusion device and the polymer melt loaded with blowingagent produced in the second zone of the extrusion device has asolidification temperature, the method being characterized in that ablowing agent concentration is set that is at least equal to a criticalminimum concentration for complete foaming, and the foaming temperatureis set to a value that is 10 to 30% higher than the solidificationtemperature.
 6. Method according to claim 5, wherein the foamingtemperature is set to a value that is 10 to 20% higher than thesolidification temperature.
 7. Method according to claim 1, wherein theblowing agent is introduced into the first zone of the extrusion deviceat a pressure that is higher than that in the first zone of theextrusion device.
 8. Method according to claim 1, wherein the polymermelt loaded with blowing agent is mixed in the second zone by means ofmixing elements.
 9. Method according to claim 1, wherein the blowingagent is CO₂.
 10. Method according to claim 1, wherein the die isconditioned to a die temperature that can be set independently of thefoaming temperature and is higher than the foaming temperature. 11.Method according to claim 10, wherein the die temperature is set to avalue that is up to 20% higher than the foaming temperature.
 12. Methodaccording to claim 1, wherein the pressure ρ_(min) in the second zone ofthe extrusion device is at least 120 bar.
 13. Method according to claim1, wherein the at least one thermoplastic polymer is at least onepolyolefin.
 14. Method according to claim 1, wherein the at least onepolyolefin is polypropylene.
 15. Method according to claim 1, whereinthe foamed polymer molded body is a solid fiber.
 16. Method according toclaim 1, wherein the foamed polymer molded body is a hollow fiber. 17.Method according to claim 1, wherein the foamed polymer molded body is afilm.
 18. Method according to claim 1, wherein the foamed structure isbroken up into foamed polymer particles.
 19. Method according to claim1, wherein the foamed structure is granulated.
 20. A foamed microporouspolymer molded body in particle form comprising at least onethermoplastic polymer with open-cell pore structure of uniformcross-sectional distribution, a porosity of 40 to 90 vol. % and anaccessible proportion of pore volume of at least 0.75, whereby the cellsconstituting the open-cell pore structure have a mean cell size ofbetween 1 and 100 μm.
 21. Foamed microporous polymer particles accordingto claim 20 with a porosity between 50 and 80 vol. %.
 22. Foamedmicroporous polymer particles according to claim 20, wherein the atleast one thermoplastic polymer is at least one polyolefin.
 23. Foamedmicroporous polymer particles according to claim 22, wherein the atleast one polyolefin is polypropylene.
 24. Foamed microporous polymerparticles according to claim 20, wherein the particles have a meanparticle size of 1 to 5 mm.
 25. Foamed microporous polymer particlesaccording to claim 20, wherein the particles have a surface porosity ofat least 25%.
 26. Foamed microporous polymer particles according toclaim 20, wherein the particles have a characteristic loading time of<90 min.
 27. Method according to claim 5, wherein the blowing agent isintroduced into the first zone of the extrusion device at a pressurethat is higher than that in the first zone of the extrusion device. 28.Method according to claim 5, wherein the polymer melt loaded withblowing agent is mixed in the second zone by means of mixing elements.29. Method according to claim 5, wherein the blowing agent is CO₂. 30.Method according to claim 5, wherein the die is conditioned to a dietemperature that can be set independently of the foaming temperature andis higher than the foaming temperature.
 31. Method according to claim30, wherein the die temperature is set to a value that is up to 20%higher than the foaming temperature.
 32. Method according to claim 5,wherein the pressure ρ_(min) in the second zone of the extrusion deviceis at least 120 bar.
 33. Method according to claim 5, wherein the atleast one thermoplastic polymer is at least one polyolefin.
 34. Methodaccording to claim 5, wherein the at least one polyolefin ispolypropylene.
 35. Method according to claim 5, wherein the foamedpolymer molded body is a solid fiber.
 36. Method according to claim 5,wherein the foamed polymer molded body is a hollow fiber.
 37. Methodaccording to claim 5, wherein the foamed polymer molded body is a film.38. Method according to claim 5, wherein the foamed structure is brokenup into foamed polymer particles.
 39. Method according to claim 5,wherein the foamed structure is granulated.